For restricted earth fault protection relay for transformer, what Class of current transformers are used. Is it 5 P 10 or class X and why?
In the existing system I find Current Transformer Class 5P10 used for REF for Transformer.
Thanks in advance.
It depends on the type of principle used for REF protection.
In case of High impedance type, Class-X (or Class-PX) is recommended.
If it is low impedance type (also known as Bias type), Class-P type is acceptable.
More details can be found in the Relay literature.